Composite Core Densification

ABSTRACT

A reinforcement tube for composite core densification and a composite article therewith.

REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. Ser. No.11/957,909, which was filed on Dec. 17, 2007.

BACKGROUND

The present invention relates to composite articles, and moreparticularly to core densification.

Some composite articles incorporate a core such as honeycomb (HC) orfoam for fabrication of various aerospace structures, such as panels,due to these core's advantageous strength to weight ratio. Such corecomposite articles include upper and lower composite skins, i.e., fiberreinforced resin matrix laminates that are separated and stabilized bythe core layer.

In areas where fasteners are to be located, the core must be densifiedby either filling the HC core with an epoxy syntactic material or theincorporation of densely packed vertical pin densification for a foamtype core. Either core densification limits fastener size to avoid corecrush.

The HC core densification may require expensive hand work and isrelatively heavy in weight. Provisions for fastener installation incomposite sandwich structures are typically accomplished with an epoxysyntactic material at 45 lbs. per cu. ft. (pcf) to fill cells in atypical 3.0 pcf HC core. HC core details are often locally densified ina separate operation that may require machining and special bondpreparation before the core detail can be assembled into a sandwichlaminate. Furthermore, densification of curved core details may requiresignificant tooling to maintain curvature during densification.

Vertical pin densification in foam type cores such as X-Cor™ or K-Cor™,is a pin insertion process typically performed at the core manufacturer.Pin densification in a 3.5 pcf or higher core is limited to 17 pcf asvertical pins may begin to interfere with X-Cor™ or K-Cor™ pins. A 17pcf vertical pin densification with 4 ply face skins limits fastenersize to approximately 5/32 inch diameter. Clamp-up loads for largerdiameter fasteners may still crush the densified core and generallyshould not be utilized with vertical pin densification. The separatevertical pin insertion operation when manufacturing the core increasesmachine time. Furthermore, at highly loaded fastener locations in flareattachment areas, vertical pin densification may not efficientlytransfer shear loads into the core.

Accordingly, it is desirable to provide localized densification oflightweight composite articles, e.g., to react significantly throughfastener load.

SUMMARY

A reinforcement tube for composite core densification according to anexemplary aspect of the present invention includes at least onecomposite ply material strip in a coil configuration, said coilconfiguration defining a tube generally rectilinear in cross-section.

A composite article according to an exemplary aspect of the presentinvention includes a core, a reinforcement tube bonded to the core and acomposite skin bonded to the core and reinforcement tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently disclosed embodiment. The drawings thataccompany the detailed description can be briefly described as follows:

FIG. 1 is a sectional view of a composite panel having an exemplaryreinforcement tube bonded therein;

FIG. 2A is a perspective view of the reinforcement tube shown in FIG. 1;

FIG. 2B is a perspective view of the reinforcement tube shown in FIG. 1illustrating the multi-directional flexibility thereof in an exemplaryembodiment;

FIGS. 3A-3D are cross-sectional views of various exemplary reinforcementtube cross-sectional shapes;

FIG. 4 is a chart illustrating an exemplary reinforcement tubemanufacturing process flow;

FIGS. 5A-5D are perspective views of steps within the manufacturingprocess flow of FIG. 4;

FIG. 6A is a cross-sectional view of a composite panel with a hollowreinforcement tube bonded therein;

FIG. 6B is a cross-sectional view of a composite panel illustrating areinforcement tube which has been filled with a lightweight expandingfoam so as, e.g., to minimize ingress of moisture;

FIG. 7A is a perspective partial phantom view of a structuremanufactured with composite panels according to an exemplary embodimentof the present invention;

FIG. 7B is a sectional view of an access panel taken along line 7B-7B inFIG. 7A illustrating a flush mount access panel arrangement; and

FIG. 7C is a cross-sectional view of a composite panel arrangement takenalong line 7C-7C in an abutted relationship as mounted to a bulkhead inthe structure of FIG. 7A.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT

Referring to FIG. 1, a lightweight core composite article 10 isillustrated in cross section. The composite article 10 includes acomposite sandwich structure having a multiple of layers bondedtogether. The composite article 10 may be manufactured in a single stepprocess using prepreg autoclave processing or resin infusion techniques,such as, for example, resin film infusion (RFI), or combinations ofthese techniques. It should be understood that various methods may beutilized to bond each layer to the adjacent layer and that variousthicknesses (e.g., number of plies) within each layer may be utilized.It should be further understood that the composite article 10 may be butone portion of a structure such as a composite panel.

The composite article 10 generally includes composite skins 14, 16 and acore 18. The composite skins 14, 16 may be bonded to the core 18 througha respective film adhesive ply 20. The composite skins 14, 16 mayalternatively be formed from uncured “prepreg” or “B-stage” laminates oforientated fiber reinforcement such as graphite, aramide or fiberglassfibers disposed in a binding matrix such as epoxy, phenolic or othersimilar organic resinous material for direct application to the core 18.The resin impregnated composite material is staged to form the (“tacky”)composite material (prepreg).

The core 18 may be a honeycomb (HC) core or a foam core material such asK-Cor™ or X-Cor™ manufactured by Aztex, Inc. of Waltham, Mass. TheK-Cor™ or X-Cor™ includes a multiple of pins within a lightweightcarrier to form a truss-like support structure. The lightweight carriermay be a Rohacell® foam which is a ‘closed cell’ type foam that isresistant to ingress by water. The pins may be manufactured of anon-metallic material such as carbon, fiberglass, quartz, Kevlar,ceramics or other material which provide desired mechanical, electricaland magnetic properties. X-Cor™ is substantially similarly to K-Cor™except X-Cor™ has sharp pin ends in order to at least partially pierceat least one ply of the composite skins 14, 16 to form a bond/mechanicallock therewith. The pins used in K-Cor™ are bent over and bonded to orbetween at least one ply of the composite skins 14, 16.

The HC core, X-Cor™ and K-Cor™ can be tailored, even within the samepanel, to accommodate various core strength and stiffness requirementsand are not restricted to that disclosed herein as any core materialwill benefit from the present invention.

Densification of the core 18 is often required in reinforcement areaswhere fasteners or preformed components transfer load into the compositearticle 10. The reinforcement area 22 provided herein includes areinforcement tube 24 (also illustrated in FIGS. 2A-2B) that is in anexemplary embodiment co-cured into the composite article 10. Thereinforcement tube 24 may be a precured segmented composite tube ofcomparable thickness to the composite article 10 sandwich structure core18. Although illustrated as generally rectilinear in cross-section inthe disclosed embodiment of FIG. 1, it should be understood that anycross-sectional shape may alternatively be utilized (FIGS. 3A-3D). Itshould be further understood that the reinforcement tube 24 mayalternatively include a metallic material.

The reinforcement tube 24 generally forms a coil configuration (FIG. 2A)about a longitudinal axis L to form a box-shaped spring in therectilinear cross-section embodiment. The coil configuration defines agenerally helical opening 25 about axis L that permits multi-directionalflexibility (FIG. 2B) along two or more axes which minimizes tooling tomaintain curvature for densification of curved core articles and therebyminimizes labor and expense.

The reinforcement tube 24 may be bonded to various types of core 18,such as, but not limited to, HC core, X-Cor™ and K-Cor™. Thereinforcement tube 24 may be bonded to the lightweight core materialwith a foaming adhesive 26 while the skin 14, 16 bond may beaccomplished with the film adhesive ply 20 such as Hysol® materialsmanufactured by Henkel Corporation headquartered in Dusseldorf, Germany.It should be understood that various foaming and film adhesives as wellas other bonding techniques may alternatively or additionally beutilized. The core mating surface 28 for a rectilinear reinforcementtube 24 may be prepared with a perpendicular edge cut. That is, the coremating surface 28 is cut in relationship to the reinforcement tubecross-sectional shape (FIGS. 3A-3D).

The reinforcement tube 24 supports the composite skins 14, 16 in thelocalized densification area to provide for load transfer to, in oneembodiment, react a fastener f load. The reinforcement tube 24essentially densifies a region of the composite article 10 such that ahole h can be formed to receive the fastener F. The fastener F, forexample, allows the composite article 10 to be readily fastened to otherstructures.

Referring to FIGS. 4, 2B, and 5A-5D, the reinforcement tube 24 isgenerally manufactured by winding a strip 30 of composite ply laminatematerial such as composite prepreg around a mandrel M (FIG. 5A) in aspiral manner to define the generally helical opening 25 (FIG. 5B);cover the laminate material with peelply, release film, and ⅛″ thicksilicone rubber; insert temperature monitoring thermocouple; cure cycleper material manufacturer recommendations; remove expendable baggingmaterials and silicone rubber strips; remove resin flash; flex thereinforcement tube 24 to break resin between the windings (FIG. 2B); andremove peelply from part outer surface and lay-up directly into sandwichlaminate of the composite article 10.

The reinforcement tube 24 is readily optimized for size, wall thickness,and lay-up configuration according to load requirements by providing anappropriate number of plies to build up the strip 30. It has beendetermined that the reinforcement tube 24, when manufactured on astraight rectilinear mandrel M, may result in a slightly twistedreinforcement tube after cure such that the mandrel M may include aslight counter-twist to produce a square reinforcement tube 24.

The generally helical opening 25 may become partially filled with resinfrom manufacture. The resin which at least partially fills the generallyhelical opening 25 readily breaks away when the reinforcement tube 24 isflexed (FIG. 2B). That is, even if the generally helical opening 25 maybe partially filled with cured resin, flexing of the reinforcement tube24 results in break-away of the resin and a flexible reinforcement tube24.

The reinforcement tube 24 may be manufactured as continuouscommodity-type material then cut to length ready to bond into a sandwichlaminate (FIG. 6A) or the desired composite article 10. That is, thereinforcement tube 24 may be manufactured in an automated manner toprovide significant lengths of particular size, wall thickness, lay-upand other configurations according to load requirements then need onlybe later cut to length.

The reinforcement tube 24 may additionally be filled with a relativelylightweight expanding foam 34 injected into an open end of thereinforcement tube 24 (FIG. 6B), e.g., to minimize the potential ofmoisture collection. In one non-limiting embodiment, the lightweightexpanding foam 34 may be less than 8.0 pcf after expansion. Furthermore,the expanding foam 34 utilized to fill the reinforcement tube 24 mayalternatively or additionally be foaming adhesive 26 to both fill thereinforcement tube 24 and expand through the generally helical opening25 to bond the reinforcement tube 24 to the core 18. The expanding foam34 and/or foaming adhesive 26 further fixes the reinforcement tube 24into the installed position. That is, the multi-directional flexibilityof the reinforcement tube 24 becomes essentially rigid when theexpanding foam 34 and/or the foaming adhesive 26 are/is cured.

In another non-limiting embodiment, the reinforcement tube 24 mayadditionally be filled with a precast foam 34 which may be slid into anopen end of the reinforcement tube 24. In this non-limiting embodiment,the precast foam 34 may be approximately 2.0 pcf.

Referring to FIG. 7A, a reinforcement tube 24′ may provide for acontinuous densification solution at a bulkhead, frame location, orother localized densification area. A multiple of reinforcement tubes24′ may be utilized to, in one disclosed embodiment, frame an opening Owhich receives a removable access panel P. The multiple of reinforcementtubes 24′ extend from the skins 14′, 16′ to facilitate flush mounting ofthe access panel P adjacent the skin 14′ (FIG. 7B). A multiple offastener apertures F_(A) are located through the panel P and themultiple of reinforcement tubes 24′ such that the panel P is removablymountable thereto.

Composite panels C with edge densification provided by reinforcementtubes 24″ may, for example, be mounted directly to a bulkhead B or suchlike with through fasteners F (FIG. 7C).

The reinforcement tube 24 of the exemplary non-limiting embodimentsdisclosed herein may provide an approximately 50 percent weight savingsover 45 lbs. per cu. ft. (pcf) epoxy syntactic; may eliminate laborintensive honeycomb core densification for fastener installation; mayallow fastener sizes beyond vertical pin densification limits; mayfacilitate fastener pitch change and fastener repositioning withoutcostly rework; and may eliminate skin build-up requirements.

It should be appreciated that composite articles may include manydifferent types of non-planar articles and planar articles such aspanels used in fixed wing aircraft, ground transportation vehicles, etcand that various panel sizes, layer combinations and depth of layers maybe utilized and specifically tailored to provide the desired panel.

It should be understood that relative positional terms such as“forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like arewith reference to the normal operational attitude of the vehicle andshould not be considered otherwise limiting.

It should be understood that although a particular component arrangementis disclosed in the illustrated embodiment, other arrangements willbenefit from the instant invention. Although particular step sequencesare shown, described, and claimed, it should be understood that stepsmay be performed in any order, separated or combined unless otherwiseindicated and will still benefit from the present invention.

The foregoing description is exemplary rather than defined by thelimitations within. Many modifications and variations of the presentinvention are possible in light of the above teachings. The disclosedembodiments of this invention have been disclosed, however, one ofordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described. For thatreason the following claims should be studied to determine the truescope and content of this invention.

What is claimed is:
 1. A reinforcement tube comprising: at least onecomposite ply material strip in a coil configuration, said coilconfiguration defining a tube generally rectilinear in cross-section. 2.The reinforcement tube as recited in claim 1, wherein said coilconfiguration provides multi-directional flexibility.
 3. Thereinforcement tube as recited in claim 1, wherein said coilconfiguration is defined about a longitudinal axis, said cross-sectiontransverse to said longitudinal axis.
 4. The reinforcement tube asrecited in claim 1, wherein said coil configuration defines a generallyhelical opening.
 5. The reinforcement tube as recited in claim 4,wherein said generally helical opening is at least partially filled witha cured resin.
 6. The reinforcement tube as recited in claim 1, furthercomprising a relatively lightweight expanding foam that at leastpartially fills said tube.